1. Field of the Invention
The present invention relates to basalt fiber that has superior sound-absorbing and heat-resistance properties. More specifically, the invention relates to inexpensive heat-resistant basalt fiber having superior sound-absorbing and heat-resistance properties that can be suitably used for automobile mufflers and the like.
2. Background Art
Automobile mufflers are components for absorbing exhaust noise. They currently employ glass fiber as a sound-absorbing material. As a result of the recent trend toward measures for saving more and more energy in automobile engines and increasingly stringent exhaust gas emission regulations, engine temperature has become higher. This has resulted in the exhaust gas temperature of 800° C. or higher at the muffler. Thus, there is an acute need to provide the sound-absorbing material in mufflers with higher heat-resistance property (to cope with temperatures of 850 to 900° C.).
In an example of the manufacture of heat-resistant glass fiber, E glass fiber is treated with an acid. Specifically, conventional E glass fiber having the composition of 50 to 63 wt. % of SiO2, 12 to 16 wt. % of Al2O3, 8 to 13 wt. % of B2O3, 15 to 20 wt. % of CaO+MgO, and trace amounts of Na2O+K2O is subjected to an immersion treatment using, e.g., a 9 to 12 wt. % concentration of hydrochloric acid at 40 to 70° C. for approximately 30 minutes to several hours. This results in the formation of silica glass in the surface layer portion having a SiO2 content of 80 percent or more by weight, thus providing the fiber with heat-resistance property.
Such acid-treated E glass fiber is advantageous in that it can be easily and cheaply spun due to the large difference between its spinning temperature and liquid-phase temperature. However, as a sound-absorbing material for the mufflers where the exhaust gas temperature can reach 700° C. or higher, the heat-resistance property of the fiber is not sufficient and impractical. While S glass fiber has high heat-resistance property and could be used as a sound-absorbing material, it is very expensive.
JP Patent Publication (Kokai) No. 2001-206733 A discloses a heat-resistant glass fiber suitable for use in the automobile mufflers as a sound-absorbing material where the exhaust gas temperature reaches 800° C. or higher. The glass fiber has a glass composition containing, for the fiber as a whole and in substantial percentage by weight terms, 56% to 58.5% of SiO2, 12 to 17% of Al2O3, 16 to 27% of CaO, 1 to 9% of MgO, 0 to 1% of Na2O, and 0 to 1% of K2O and not containing B2O3 nor F2, where the surface layer portion is formed of silica glass containing 90 wt. % or more of SiO2. The patent document also discloses a heat-resistant glass fiber obtained by subjecting the surface of the glass fiber of the aforementioned composition to an acid treatment with mineral acid.
Meanwhile, basalt filament, which is made from naturally occurring basalt rock ore, is very inexpensive as compared with conventional glass filament. However, when it is used at high temperatures of approximately 750° C. to 900° C., crystalline phase develops from the glass component, resulting in the disappearance of flexibility, peeling at the interface between the crystalline layer and the glass layer, and other problems.
Specifically, these problems are:
(1) Commercially available glass fiber that consists primarily of SiO2, Al2O3, and CaO suffers from sound-absorbing and durability properties problems when exposed to high-temperature (approximately 800° C.) gas in the exhaust system.
(2) Commercially available glass fiber that consists primarily of SiO2, Al2O3, and MgO suffers from the sound-absorbing and durability properties problems when exposed to high-temperature (approximately 830° C.) gas in the exhaust system.
(3) Commercially available glass filament is expensive.
(4) Application of basalt fiber using naturally occurring material is being considered to solve the above problems (1) to (3). Typical examples are the two kinds of basalt fiber that are compositionally stable and available in large quantities, namely, an ore (A, for high-temperature applications) having a larger amount of SiO2 and an ore (B, for intermediate-temperature applications) having a smaller amount of SiO2, both of which have approximately the same amount of Al2O3. Although basalt fiber using ore A as a raw material can be spun into fiber, the resultant basalt fiber has a heat-resistance problem at temperature ranges exceeding 750° C. Ore B, on the other hand, is associated with higher energy cost for mass production of fiber.
(5) Thus, no sound-absorbing glass fiber or heat-insulating component material for automotive applications that satisfies the requirements for heat resistance, low cost, and high durability has been available.
JP Patent Publication (Kokai) No. 2001-315588 A discloses an invention whereby basalt fiber is added in a resin for automobile interior material.